Putative nicotinic and muscarinic receptors have been identified in membranes from housefly brain by means of the characteristics of their binding of specific drugs, (125I)alpha-bungarotoxin ((125I)alpha-BGT) for the nicotinic and (3H)quinuclidinyl benzilate ((3H)QNB) for the muscarinic receptors. The kinetics of binding of (125I)alpha-BGT to membrane preparations isolated from housefly head homogenates were studied and found to be similar to those of vertebrate nicotinic ACh-receptors. Also, their pharmacologic selectivities (evaluated by studying the effect of various drugs on the initial rate of (125I)alpha-BGT binding) were essentially similar. The kinetics of the specific binding of (3H)QNB to the same membrane preparation and the sensitivity of this binding to cholinergic drugs have been studied. The results imply that housefly brains contain muscarinic receptors as well as the nicotinic ones. Work is in progress now to determine the affinities of agonists and antagonists of the muscarinic receptor for better comparison of these receptor sites with their counterparts in the mammalian brain and smooth muscles. We also detected binding of (3H)perhydrohistrionicotoxin to the housefly brain membranes. This toxin is used as a probe for the ionic channel associated with the fish electric organ nicotinic receptor. The number of its binding sites in housefly brain membranes (33.3 plus or minus 5.5 pmoles/g heads) is similar to that of (125I)alpha-BGT (27.8 plus or minus 3.2 pmoles/g heads) in the same membranes. The binding of (3H)perhydrohistrionicotoxin was inhibited only by drugs that were found to interact with the ionic channel of the nicotinic receptor, but not by receptor agonists or antagonists, which suggests that this binding may be to ionic channels associated with the nicotinic ACh-receptors in housefly brain.